The invention relates to The invention relates to an internal combustion engine having an exhaust-gas purification system with a nitrogen oxide storage catalytic converter and an SCR catalytic converter connected downstream of the nitrogen oxide storage catalytic converter, in which the nitrogen oxide storage catalytic converter can be supplied in three operating modes with excess oxidizing constituents, excess reducing constituents, and lower levels of both constituents. The invention also relates to a related method for purifying the exhaust gas from an internal combustion.
Laid-Open Specification DE 101 13 947 A1 has disclosed an exhaust-gas purification system for an internal combustion engine having a nitrogen oxide storage catalytic converter, in which, in regeneration phases of the nitrogen oxide storage catalytic converter, ammonia (NH3) is released by the latter and is used to remove nitrogen oxides (NOx) in an SCR catalytic converter connected downstream as seen in the direction of flow. In the corresponding exhaust-gas purification method, NOx is removed from the exhaust gas during a lean-burn mode of the internal combustion engine by being accumulated in the catalyst material of the nitrogen oxide storage catalytic converter. Once it is saturated, the nitrogen oxide storage catalytic converter is regenerated during a rich engine operating mode. According to DE 101 13 947 A1, during this regeneration in a first phase an exhaust gas which is relatively highly enriched with reducing agents is supplied to the nitrogen oxide storage catalytic converter. In a second phase of the regeneration, by contrast, an exhaust gas with a lower reducing agent content is supplied to the nitrogen oxide storage catalytic converter. NOx which has accumulated in the catalyst material of the nitrogen oxide storage catalytic converter is reduced to form NH3, which is supplied to the downstream SCR catalytic converter and accumulated therein. During the lean operating phase which follows the regeneration, NOx which passes through the nitrogen oxide storage catalytic converter can be selectively reduced in the SCR catalytic converter. The NH3 which has previously been accumulated in the SCR catalytic converter serves as a selectively acting reducing agent. In this way, the nitrogen-oxide-lowering properties of the nitrogen oxide storage catalytic converter and of the SCR catalytic converter complement one another. However, a sufficient supply of NH3 to the SCR catalytic converter is important for efficient reduction of the levels of NOx.
It is an object of the invention to provide an internal combustion engine having an exhaust-gas purification system and a method for purifying the exhaust gas from an internal combustion engine of the type described in the introduction, with which overall the highest possible nitrogen oxide purification action is achieved.
The internal combustion engine according to the invention has an exhaust gas purification system comprising a nitrogen oxide storage catalytic converter and an SCR catalytic converter connected downstream of the nitrogen oxide storage catalytic converter, wherein the nitrogen oxide storage catalytic converter can be supplied in a first operating mode with exhaust gas containing an excess of oxidizing constituents and in a second operating mode with exhaust gas containing an excess of reducing constituents. According to the invention, a third operating mode is provided, in terms of time, after the first operating mode and before the second operating mode, in which third operating mode the nitrogen oxide storage catalytic converter can be supplied with an exhaust gas which has a lower content of oxidizing constituents than the first operating mode and a lower content of reducing constituents than the second operating mode.
It is known that the chemical reduction of NOx to form NH3 requires an environment with a chemically reducing action. As has surprisingly been found, however, even relatively small residual quantities of oxidizing constituents and in particular of oxygen have a lasting adverse effect on the efficiency of NH3 formation even in relatively strongly reducing conditions. Since the nitrogen oxide storage catalytic converter used is either a honeycomb body with passages passing through it or a bulk bed of shaped bodies, in the event of a sudden change in the exhaust-gas composition from oxidizing to reducing, the exhaust gases of different compositions become mixed with one another in the cavities formed by these catalyst converter structures. This mixing briefly results in relatively powerful reactions, impeding the reduction of NOx accumulated in the catalyst material to form NH3, on account of the oxygen still being present. Instead, NOx can even be suddenly released and then leave the nitrogen oxide storage catalytic converter without being reduced, in the form of what is known as NOx breakthrough. This has an adverse effect on the purifying action of the exhaust-gas purification system. Conversely, in particular at the beginning of nitrate regeneration, when relatively large quantities of NOx are still stored in the catalyst material of the nitrogen oxide storage catalytic converter, effective NOx reduction with a high level of NH3 being formed is desirable. This is achieved by a third operating mode, in which the nitrogen oxide storage catalytic converter is supplied with an exhaust gas which has a lower oxygen content than the first operating mode and a lower reducing agent content than the second operating mode, being established in terms of time after the first operating mode and before the second operating mode.
In the third operating mode, the gas of the first operating mode which is present in the cavities in the catalytic converter body is replaced by a gas which has a lower content of highly reactive constituents than the first operating mode and the second operating mode. This avoids the undesirable effects which have been mentioned above and creates conditions in the nitrogen oxide storage catalytic converter which improve the formation of NH3 in the subsequent second operating mode. Therefore, the SCR catalytic converter connected downstream of the nitrogen oxide storage catalytic converter can be supplied with a relatively large quantity of NH3, which correspondingly improves its efficiency. In particular, in the third operating mode the oxygen content of the exhaust gas which is present in the nitrogen oxide storage catalytic converter is lowered, and as a result the NH3 yield is improved during the NOx reduction in the storage catalytic converter.
The setting of the exhaust-gas composition in the individual operating modes can be performed by the internal combustion engine, which may be implemented as a diesel engine or as a spark-ignition engine and has suitable control devices which allow corresponding internal combustion engine operating modes. However, the setting of the exhaust-gas composition can crucially also be performed or assisted by a gas-delivering additional device in at least one of the operating modes.
In one configuration of the invention, the nitrogen oxide storage catalytic converter is designed as an arrangement of a first nitrogen oxide storage catalytic converter element and a second nitrogen oxide storage catalytic converter element which is connected parallel in terms of flow with the first nitrogen oxide storage catalytic converter element. This gives rise to the possibility of actuating the two nitrogen oxide storage catalytic converter elements separately and operating them at offset times in the individual operating modes. This can likewise improve the supply of NH3 to the SCR catalytic converter connected downstream of the nitrogen oxide storage catalytic converter elements. The separate actuation of the nitrogen oxide storage catalytic converter elements can be achieved, for example, by enabling them to be connected to different cylinders of the internal combustion engine and the cylinders being operated with different air/fuel mixes.
In a further configuration of the invention, the first nitrogen oxide storage catalytic converter element and the second nitrogen oxide storage catalytic converter element can be operated alternately either in the first operating mode or in the second operating mode and third operating mode. Therefore, by way of example, the first nitrogen oxide storage catalytic converter element is acted on by oxidizing exhaust gas from lean-burn cylinders, while the second nitrogen oxide storage catalytic converter element is operated in the third or second operating mode and is therefore acted on by low-oxygen or reducing exhaust gas.
In a further configuration of the invention, a switching device is provided, in such a manner that the nitrogen oxide storage catalytic converter element which is operating in the second operating mode and/or in the third operating mode can be at least partially isolated from the exhaust-gas stream released from the internal combustion engine. As a result, the exhaust-gas stream passing through the nitrogen oxide storage catalytic converter element which is being operated in the second operating mode and/or in the third operating mode can be lowered to a greater or lesser extent, which facilitates the change in the exhaust-gas composition in the nitrogen oxide storage catalytic converter element, since a smaller quantity of gas is affected. The switching device may in this case be designed, for example, as an exhaust-gas flap which diverts the exhaust-gas stream and is connected upstream of the nitrogen oxide storage catalytic converter elements.
In a further configuration of the invention, a gas delivery device is provided, in such a manner that the nitrogen oxide storage catalytic converter, which is operated in the second operating mode and/or in the third operating mode, can be acted on by a gas stream delivered by the gas delivery device. This configuration of the invention allows the depletion of the oxygen content in the exhaust gas or the enrichment of the reducing agent content in the exhaust gas for the nitrogen oxide storage catalytic converter which is being operated in the third and/or second operating mode to be performed at least in part by the gas delivery device. This can make the change in internal combustion engine operation more moderate.
The gas delivery device provided may be a single cylinder or a plurality of combined cylinders of the internal combustion engine or an external unit. The latter case is advantageous, in particular with a parallel connection of two nitrogen oxide storage catalytic converter elements, if the nitrogen oxide storage catalytic converter element which is affected by the gas change can be completely or partially isolated from the main exhaust-gas stream from the internal combustion engine. If appropriate, it is then even possible to completely dispense with a change in internal-combustion engine operation and to enable the internal combustion engine to be continuously operated in lean-burn mode. The change in the gas composition in the third or second operating mode compared to the first operating mode is in this case effected exclusively by the gas delivery unit.
In a further configuration of the invention, a low-oxygen gas stream can be delivered by the gas delivery device. The gas delivery device preferably enables a low-oxygen gas with a different reducing agent content to be delivered. Therefore, the oxygen depletion of the exhaust gas which flows through the nitrogen oxide storage catalytic converter operated in the third operating mode is carried out predominantly or completely by the gas delivery device. Similarly, the reducing agent depletion of the exhaust gas in the second operating mode can likewise be performed by the gas delivery device.
In a further configuration of the invention, the gas delivery device is designed as a fuel reformer or as a burner. It is preferable for the fuel reformer or the burner to be operated with the fuel of the internal combustion engine. The fuel preparation carried out in the gas delivery unit can in this case be catalytically assisted.
In a further configuration of the invention, a catalytic converter element with an oxidation catalytic action is connection upstream of the nitrogen oxide storage catalytic converter. By way of example, an oxidation catalytic converter or a three-way catalytic converter is suitable for use as this catalytic converter element. The catalytic converter element with an oxidation catalytic action catalyzes the reaction of reducing agents with oxygen, so that an excess of oxygen or reducing agents in the exhaust gas can be lowered. Therefore, the result of this configuration of the invention is that in the third operating mode the nitrogen oxide storage catalytic converter receives a relatively inert exhaust gas, so that the formation of NH3 in the subsequent third operating mode is not impeded by excess oxygen.
In a further configuration of the invention, a particulate filter is connected upstream of the SCR catalytic converter. Therefore, the exhaust-gas purification action of the exhaust-gas purification installation, in addition to lowering the nitrogen oxide levels, also comprises lowering the particulate levels, which is advantageous in particular in the case of an internal combustion engine in the form of a diesel engine. The particulate filter may be arranged immediately upstream of the SCR catalytic converter or may also be connected upstream of the nitrogen oxide storage catalytic converter.
The method according to the invention for purifying the exhaust gas from an internal combustion engine provides that a nitrogen oxide storage catalytic converter is supplied in a first method step with exhaust gas containing an excess of oxidizing constituents, with nitrogen oxides being removed from the exhaust gas by being accumulated in the nitrogen oxide storage catalytic converter, in a second method step with exhaust gas containing an excess of reducing constituents, with nitrogen oxide which has been accumulated in the nitrogen oxide storage catalytic converter being at least partially reduced to NH3, and in a third method step, which in terms of time is carried out after the first method step and before the second method step, with an exhaust gas which has a lower content of oxidizing constituents than in the first method step and a lower content of reducing constituents than in the second method step. In the third method step, therefore, the feed lines leading to the nitrogen oxide storage catalytic converter and also the nitrogen oxide storage catalytic converter itself are purged with a virtually inert exhaust gas, and the relatively high oxygen content of the exhaust gas in the cavities in the catalytic converter is lowered. As a result, during the transition to the second method step, reactions which take place in the nitrogen oxide storage catalytic converter are much less powerful, and favorable conditions are preset for the NH3 formation in the second method step in the nitrogen oxide storage catalytic converter.
In one configuration of the method, the third method step is terminated at the earliest when the nitrogen oxide storage catalytic converter has been predominantly filled by exhaust gas delivered in the third method step. In this context, the filling of the catalytic converter is to be understood as meaning the filling of the cavities which are present therein. This ensures that the gas column with a relatively high oxygen content which results from the first method step is predominantly flushed out of the nitrogen oxide storage catalytic converter.
In a further configuration of the method, in the case of a nitrogen oxide storage catalytic converter formed as a parallel arrangement of a first nitrogen oxide storage catalytic converter element and a second nitrogen oxide storage catalytic converter element, the first nitrogen oxide storage catalytic converter element and the second nitrogen oxide storage catalytic converter element are operated alternately, via a switching device, in the first method step or in the second and third method steps. As a result, a continuous operation of the exhaust-gas purification system with regard to the removal of NOx by accumulation in the catalyst material of the nitrogen oxide storage catalytic converter and by reduction in the SCR catalytic converter is achieved.
In a further configuration of the method, the exhaust gas which is supplied to the nitrogen oxide storage catalytic converter in the second method step and/or in the third method step is at least partially delivered by a gas delivery unit which is designed as a fuel reformer or as a burner. As a result, there is no need for a very major change in the air/fuel ratio when the internal combustion engine is operating, and the internal combustion engine can if appropriate even be operated continuously under lean-burn conditions, because the change in the composition of the exhaust gas which flows through the nitrogen oxide storage catalytic converter is in part effected by the gas delivery unit.
In a further configuration of the method, in the second and third method steps the oxygen content of the exhaust gas is catalytically lowered upstream of the nitrogen oxide storage catalytic converter. For this purpose, a catalytic converter element with an oxidation catalyst action is preferably connected upstream of the nitrogen oxide storage catalytic converter. One advantage of this configuration of the method is that the heat of reaction which is released at this catalytic converter element can be utilized to increase the temperature of the nitrogen oxide storage catalytic converter connected downstream.
In a further configuration of the method, the temperature of the nitrogen oxide storage catalytic converter element is influenced, according to the temperature dependency of its efficiency, by adjusting the switching device. In the case of an internal combustion engine which is operated in lean-burn mode, it is preferable for the switching device, which is configured for example as an exhaust-gas flap, to be actuated in such a way that a predeterminable fraction of the oxidizing exhaust gas is passed into the exhaust-gas branch in which the nitrogen oxide storage catalytic converter element which is being operated in the third or second method step is arranged. Reaction of oxygen with reducing agents releases heat of reaction which is utilized to heat the components connected downstream. This can lead to these component being operated in a temperature range of optimum efficiency.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings for example.